Deprecate /plugins directory

What does this MR do?

Deprecate /plugins directory. For #196846 (closed)

Depends on !29675 (merged).

Feature rename happened earlier at !22979 (merged)

Screenshots

Admin view

Rake task

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Separation of EE specific content